Frequency measuring device, polishing device using the same and eddy current sensor

ABSTRACT

An eddy current sensor and a method for detecting the thickness of a film formed on a substrate. The eddy current sensor, which is capable of stable operation, is operable to accurately detect a polishing endpoint. The eddy current sensor detects the thickness of a conductive film from a change in an eddy current loss generated in the conductive film. The eddy current sensor comprises a sensor coil for generating an eddy current in the conductive film, and an active element unit connected to the sensor coil for oscillating a variable frequency corresponding to the eddy current loss. The sensor coil and active element unit are integrated to form the eddy current sensor. Alternatively, the eddy current sensor comprises a sensor coil for generating an eddy current in the conductive film, and a detector for detecting a change in the thickness of the conductive film from a change in a resistance component in an impedance formed by the sensor coil and conductive film.

[0001] This application is a Divisional application of Ser. No.09/982,025, filed Oct. 19, 2001, now pending.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a frequency measuring devicecapable of measuring a frequency highly accurately and continuouslywithin a short period and to a device and method for polishing by usingthe frequency measuring device.

[0003] Further, the present invention relates generally to an eddycurrent sensor, and more particularly, to an eddy current sensor whichis capable of detecting an eddy current loss produced in a conductivefilm made of copper (Cu) or the like deposited on the surface of asubstrate such as a semiconductor wafer, when the conductive film ispolished by a chemical mechanical polishing (CMP) technique, to detectthe advancement of polishing, and a method of detecting a polishedthickness using the eddy current sensor.

[0004] A chemical mechanical polishing (CMP) process has been widelyemployed as a method in which a semiconductor substrate is dipped in aplating solution to conduct, for example, electrolytic plating ornon-electrolytic plating to form an electrically conductive film, andthereafter an unnecessary electrically conductive film on the surface ofthe semiconductor substrate is removed. Hereinafter, with reference toFIGS. 1 to 3, a description will be given of the structure and operationof a polishing device for the CMP process proposed by the applicant ofthe subject application. FIG. 1 is a longitudinal cross-sectional viewshowing the entire structure of the polishing device. The polishingdevice is equipped with a turn table 101, and a top ring 102 thatpresses a semiconductor wafer 103 against the turn table 101 whileretaining the same. The turn table 101 is coupled to a motor 104 so asto be rotatable about a shaft center of the motor 104 as indicated by anarrow. A polishing cloth 105 is attached to the top surface of the turntable 101.

[0005] The top ring 102 is coupled to a motor and an elevating cylinder(not shown) through a top ring shaft 106. With this structure, the topring 102 can be elevated along the top ring shaft 106 in a directionindicated by an arrow and is rotatable about the top ring shaft 106 sothat the semiconductor wafer 103 can be pressed against the polishingcloth 105 with an arbitrary pressure. An elastic mat 107 made ofpolyurethane or the like is disposed on the lower surface of the topring 102. A guide ring 108 for latching the semiconductor wafer 103 isdisposed on the outer peripheral portion of the lower portion of the topring 102.

[0006] A polishing abrasive solution nozzle 109 is located above theturn table 101, and a polishing abrasive solution Q is supplied to thepolishing cloth 105 stuck onto the turn table 101 by the polishingabrasive solution nozzle 109.

[0007] As shown in FIG. 1, an eddy current sensor 110 that constitutesan end point detecting mechanism for detecting an end point in thepolishing of the semiconductor wafer 103 is embedded within the turntable 101. A wiring 111 of the eddy current sensor 110 passes throughthe turn table 101 and a turn table support shaft 112 and is thenconnected to a controller 114 through a rotary connector (or slip ring)113 disposed at a shaft end of the turn table support shaft 112. Thecontroller 114 is connected to a display device (display) 115. Notethat, instead of embedding the eddy current sensor 110 within the turntable 101, the eddy current sensor 110 may be embedded within the topring 102. In the case where the eddy current sensor 110 is embeddedwithin the top ring 102, the connection between the eddy current sensor110 and the controller 114 is effected through a slip ring (not shown)disposed at an appropriate position of the top ring shaft 106.

[0008]FIG. 2 is a plan view showing the positional relationship of theturn table 101, the semiconductor wafer 103 and the eddy current sensor110 in the polishing device shown in FIG. 1. As shown in the figure, theeddy current sensor 110 is located at a position that passes through acenter Cw of the semiconductor wafer 103 that is held by the top ring102 while being polished. Reference symbol Ct indicates the center ofrotation of the turn table 101. The eddy current sensor 110 sequentiallydetects the thickness of the electrically conductive film such as a Culayer of the semiconductor wafer 103 on a passing locus during a periodwhere the eddy current sensor 110 passes below the semiconductor wafer103.

[0009]FIGS. 3a and 3 b are enlarged main-portion cross-sectional viewsshowing a state where the eddy current sensor 110 is embedded. FIG. 3ashows a case where the polishing cloth 105 is attached to the turn table101, and FIG. 3b shows a case where a fixed abrasive grain plate 105′ islocated on the turn table 101. In the case where the polishing cloth 105is attached to the turn table 101 as shown in FIG. 3a, the eddy currentsensor 110 is embedded within the turn table 101. On the other hand, inthe case where the fixed abrasive grain plate 105′ is located on theturn table 101 as shown in FIG. 3b, the eddy current sensor 1 isembedded within the fixed abrasive grain plate 105′.

[0010] In either one of FIGS. 3a and 3 b, a distance L from thepolishing surface which is the top surface of the polishing cloth 105 orthe polishing surface which is the top surface of the fixed abrasivegrain plate 105′, namely, the surface (lower surface) of thesemiconductor wafer 103 to be polished, to the top surface of the eddycurrent sensor 110 may be set to 1.3 mm or more. In FIGS. 3a and 3 b,there is shown the semiconductor wafer 103 where an electricallyconductive film 103 b consisting of a Cu layer or an Al layer is formedon an oxide film (SiO₂) 103 a.

[0011] The polishing cloth 105 is made of, for example, non-woven fabricsuch as Politex manufactured by Rodale Corp. or foam polyurethane suchas IC 1000. Also, the fixed abrasive grain plate 105′ is formed bysolidifying fine abrasive grains of several μm or less in the degree ofgrains (for example, CeO₂) using resin as a bonding agent and formingthem into a disk shape.

[0012] In the polishing device shown in FIGS. 1, 2 and 3 a, thesemiconductor wafer 103 is held on the lower surface of the top ring102, and the semiconductor wafer 103 is pressed against the polishingcloth 105, or against the fixed abrasive grain pack 105′ of FIG. 3b, onthe top surface of the rotating turn table 101 by the elevatingcylinder. By supplying the polishing abrasive solution Q from thepolishing abrasive solution nozzle 109, the polishing abrasive solutionQ is retained on the polishing cloth 105, and polishing is conducted ina state where the polishing abrasive solution Q exists between thesurface (lower surface) of the semiconductor wafer 103 to be polishedand the polishing cloth 105. The eddy current sensor 110 passesimmediately below the surface of the semiconductor wafer 103 to bepolished each time the turn table 101 makes one rotation. In this case,because the eddy current sensor 110 is located on the locus that passesthrough the center Cw of the semiconductor wafer 103, the eddy currentsensor 110 can continuously detect the thickness of the electricallyconductive film on the arcuate locus of the surface of the semiconductorwafer 103 in accordance with the movement of the eddy current sensor110. Note that, in order to reduce the detection interval, indicated bythe imaginary line of FIG. 2, other eddy current sensors 110′ may beadded so that two or more eddy current sensors 110′ are disposed on theturn table 101.

[0013] A brief description will be given here of the principle on whichthe thickness of the electrically conductive film on the semiconductorwafer 103 formed of a Cu layer or an AL layer is detected by using theeddy current sensor 110 to judge the end point of the CMP process. Whena high frequency current flows through the sensor coil of the eddycurrent sensor 110 to generate an eddy current in the electricallyconductive film (metal film) of the semiconductor wafer 103, the eddycurrent sensor 110 and the semiconductor wafer 103 are magneticallycoupled to each other by a mutual inductance. In this state, since thesynthetic impedance of the sensor circuit of the eddy current sensor 110and the conductive film of the semiconductor wafer 103 becomes afunction of the resonance frequency of an oscillating circuit within theeddy current sensor 110, the resonance frequency is monitored to therebymake it possible to detect the end point in polishing.

[0014] Assume an equivalent circuit of the eddy current sensor 110 andthe semiconductor wafer 103, wherein the inductance of the eddy currentsensor 110 is L1, the resistance of the eddy current sensor 110 is R1,the inductance of the semiconductor wafer 103 is L2, the resistor of theeddy current sensor 110 is R2, and the mutual inductance thatmagnetically couples both of the eddy current sensor 110 and thesemiconductor wafer 103 is M. Also, assuming that j is the square rootof −1 (imaginary number), f is the resonance frequency of the eddycurrent sensor 110, and Ω=2πf, then the above synthetic impedance Z isrepresented as follows:

Z=(R1+Rx·R2)+j Ω)(L1−Rx·L2)

Rx=Ω ² M ²/(R2²+Ω² L2²)

[0015] Therefore, the synthetic impedance Z changes with a change in Rxand the resonance frequency f of the eddy current sensor 110 alsochanges simultaneously. By monitoring the degree of the change in theresonance frequency f, it is possible to judge the end point of the CMPprocess.

[0016] A detection signal outputted from the eddy current sensor 110 isprocessed by the controller 114 while the semiconductor wafer 103 ispolished by the polishing device shown in FIG. 1 on the basis of theabove-mentioned principle, and a change in the resonance frequency fobtained as a result of the above processing is observed. FIG. 4 showsan example of a graph in which an abscissa axis represents a polishingtime (sec.), and an ordinate axis represents the resonance frequency f(KHz) of the eddy current sensor 110, respectively. Therefore, the graphof FIG. 4 shows a change in the resonance frequency f when the eddycurrent sensor 110 passes immediately below the semiconductor wafer 103a plurality of times. Note that the electrically conductive film on thesemiconductor wafer 103 when the above graph is obtained is formed of aCu layer.

[0017] As shown in FIG. 4, as polishing is advanced, that is, as thethickness of the conductive film is reduced, the resonance frequency fobtained as a result of processing the detection signal of the eddycurrent sensor 110 by the controller 114 continues to reduce (FIG. 4shows that the resonance frequency f is gradually reduced from 6800 Hz).Therefore, by obtaining in advance the resonance frequency when theconductive film other than the wiring conductors is removed from thesemiconductor wafer 103, the resonance frequency f of the eddy currentsensor 110 is graphed as shown in FIG. 4, and the end point of the CMPprocess can be detected from a change in the inclination of theresonance frequency at the respective polishing times. In the graphshown in FIG. 4, the resonance frequency when the unnecessary conductivefilm is removed from the upper surface of the semiconductor wafer 103 is6620 Hz.

[0018] Also, it is possible to set a certain frequency before thefrequency reaches the end point of polishing that is set as a thresholdvalue, and to perform polishing by using the fixed abrasive grain plate105′ (refer to FIG. 3b) with a high polishing speed (polishing rate)until the frequency reaches the threshold value. Polishing is performedby using the polishing cloth 105 (refer to FIG. 3a) with a lowerpolishing rate than that of the fixed abrasive grain plate 105′ afterthe frequency reaches the threshold value, and the CMP process can becompleted when the frequency reaches the frequency at the time theunnecessary conductive film is removed from the upper surface of thesemiconductor wafer 103.

[0019] In this way, in the CMP process, when the unnecessaryelectrically conductive film on the semiconductor wafer is removed, ifthe conductive film is over-polished, a conductor for forming a wiringcircuit is also removed, which results in a semiconductor wafer beingproduced that cannot be used. Also, since an insufficient polishing ofthe electrically conductive film cannot remove the conductive films thatshort-circuit between the wiring circuits, it is necessary to furthercontinue polishing, thereby resulting in an increase in themanufacturing cost. Under the above circumstances, there is a requiredmeans for detecting the thickness of the electrically conductive filmformed on the polished surface of the semiconductor wafer continuouslyand highly precisely at a real time to accurately determine the endpoint of polishing.

[0020] In order to detect such a change in the resonance frequency, theoutput of the eddy current sensor is usually applied to a frequencymeasuring device. FIG. 5a schematically shows the structure of afrequency measuring device FC of a direct counting system conventionallyused as an example of the above frequency measuring device, and FIG. 5bshows a signal waveform in FIG. 5a.

[0021] In FIG. 5a, a measured signal Vin having a certain frequency issupplied to a signal input terminal IN. The measured signal Vin is apulse wave obtained by TTL-level converting a sine waveform signal, andhas such a shape, for example, as shown by Vin of FIG. 5b. After themeasured signal Vin has been amplified by an amplifier A, the measuredsignal Vin is supplied to one input terminal of a gate circuit G. A timereference signal T outputted from a time reference circuit TR issupplied to the other input terminal of the gate circuit G. As shown inFIG. 5b, the duration of the time reference signal T is t, and the timereference circuit TR supplies the time reference signal T to the gatecircuit G at given intervals. As a result, the gate circuit G is openedonly during a period of the duration when the time reference signal T issupplied, and the measured signal Vin that has passed through the gatecircuit G in a period at which the gate circuit G is opened is appliedto a decimal counter DC so as to measure the frequency of the measuredsignal Vin. The measured result is latched by a latch circuit LC andthen supplied to a given processing circuit such as a CPU.

[0022] Assuming that the number of pulses of the measured signal Vinthat has passed through the gate circuit G in a period of the duration twhen the gate circuit G is open is c, the frequency f (Hz) of themeasured signal Vin at this time is represented as follows:

f=c/t

[0023] In the case where the frequency measurement having fivesignificant digits is intended to be conducted in a conventionalfrequency measuring device shown in FIG. 5a, the duration t of the timereference circuit TR can be set to the order of 100 ms, and the timereference circuit TR outputs the time reference signal T whose durationt is, for example, 390 ms at intervals of 390 ms. This means that themeasured results of the frequency can be obtained every 390 ms, andtherefore the conventional eddy current sensor can detect a change inthe thickness of the electrically conductive film on the semiconductorwafer at the intervals of 390 ms.

[0024] However, the actual speed of the CMP process is increasing, andin order to accurately detect the end point in polishing and improve theyield, there is a demand for a device and method in which the result ofthe frequency measurement is supplied at intervals of less than 390 msto make it possible to more accurately detect the end point in polishingin the present circumstances.

[0025] As described above, in order to form a wiring circuit on asemiconductor substrate, a method has been proposed in which grooves forwires of a predetermined pattern are previously formed and the substrateis immersed in a plating solution, for example, for electroless platingor electrolytic plating of copper (Cu), and subsequently unnecessaryportions of the plated copper is removed from the surface by a chemicalmechanical polishing (CMP) process. Such deposition of a copper layer byplating enables the wiring grooves of a high aspect ratio to beuniformly filled with a highly conductive metal. The CMP processinvolves pressing a semiconductor wafer held by a top ring onto anabrasive cloth adhered on a turn table, and simultaneously supplying apolishing solution containing abrasive grain to polish a Cu layer on thesemiconductor wafer.

[0026] When the Cu layer is polished by the CMP process, the Cu layermust be selectively removed from the semiconductor substrate whileleaving only portions of the Cu layer which are formed in the groovesfor wiring. Specifically, except for the grooves for wiring, the Culayer must be removed until an oxide film (SiO₂) is exposed. In thisevent, if the Cu layer is excessively polished to remove the Cu layer inthe grooves for wiring together with the oxide film (SiO₂), a resultingincrease in circuit resistance would lead to the abandonment of theentire semiconductor substrate, thereby sustaining a great damage. Onthe other hand, the Cu layer remaining on the oxide film due toinsufficient polishing would fail to fully separate wiring circuits,thereby resulting in short-circuiting. Consequently, the polishingprocess must be performed again, thereby causing an increase inmanufacturing cost. This situation is not limited to the Cu layer but islikewise possible with any other conductive film such as an Al filmwhich is formed and polished by the CMP process.

[0027] To address the foregoing problem, a method of detecting apolishing end point using an eddy current sensor has been proposed fordetecting an end point for a CMP process. FIG. 6 illustrates a mainportion of a conventional polishing apparatus which comprises an eddycurrent sensor. The illustrated polishing apparatus comprises arotatable turn table 121 having adhered thereon an abrasive cloth 122which has a polishing surface on the top of the abrasive cloth 122; atop ring 123 for rotatably holding a semiconductor wafer 124, which is asubstrate subjected to polishing, to press the semiconductor wafer 124onto the abrasive cloth 122; and a polishing solution supply nozzle 125for supplying the abrasive cloth 122 with a polishing solution Q. Inplace of the abrasive cloth 122, an abrasive plate formed of a resinhaving abrasive grains, called “fixed abrasive grains,” may be adheredon the turn table 121. The top ring 123 is coupled to a top ring shaft126 and comprises an elastic mat 127 made of polyurethane or the likeapplied on its bottom such that the semiconductor wafer 124 is held incontact with the elastic mat 127. The top ring 123 further comprises acylindrical retainer ring 128 around its outer periphery for preventingthe semiconductor wafer 124 from coming off the bottom of the top ring123 during polishing. The retainer ring 128 is fixed to the top ring123, with its lower surface formed protrusively from a holding surfaceof the top ring 123, such that the semiconductor wafer 124 is held inthe holding surface and prevented from coming out of the top ring 123 byan abrasive force with the abrasive cloth 122 during the polishing.

[0028] The top ring 123 also comprises an eddy current sensor coil 129embedded therein which is connected to an active element unit 130, whichcomprises an oscillator circuit, through a wire 131 which extendsthrough the top ring shaft 126, and is further connected to a processor132 through an interface board 133 including a filter circuit, and adistribution box 134 including a waveform converter circuit. In thedistribution box 134, an oscillating signal is converted to a TTL level(0-5 volts), and the oscillating frequency is counted by a frequencycounter within the processor 132. The oscillating frequency thus countedis displayed on a display unit 135. The eddy current sensor coil 129 andthe active element unit forms an eddy current sensor.

[0029] The semiconductor wafer 124 is held below the elastic mat 127 onthe bottom of the top ring 123, and the semiconductor wafer 124 ispressed onto the abrasive cloth 122 on the turn table 121 by the topring 123, while the turn table 121 and the top ring 123 are rotated topolish the semiconductor wafer 124 with the abrasive cloth 122 throughrelative movements therebetween. In this event, the polishing solution Qis supplied onto the abrasive cloth 122 from the abrasive solutionsupply nozzle 125. The abrasive solution, suitable for polishing Cu(copper), for example, may be an oxidizer suspended with abrasive grainscomprising fine particles such as alumina or silica. The semiconductorwafer is polished by a combination of the action of the polishingsolution which oxidizes the surface of the Cu layer through a chemicalreaction and the action of the mechanical polishing action provided bythe abrasive grains.

[0030] During the polishing, the eddy current sensor 129, 130 continuesto detect a change in the thickness of a conductive film such as the Culayer formed on the polished surface of the semiconductor wafer 124.Then, a signal from the eddy current sensor 129, 130 is monitored todetect an end point for the CMP process, relying on a change infrequency which occurs when the conductive film on an oxide film (SiO₂)has been removed while leaving only the conductive material such as theCu layer formed in grooves for wiring.

[0031] As mentioned above, the eddy current sensor is comprised of thesensor coil 129 which is positioned opposite to a substrate subjected topolishing, and the oscillator circuit (active element unit) 130connected to the sensor coil 129 and including a capacitance and anactive element. As the active element unit 130 is supplied with DCpower, the sensor coil 129 and capacitance form a tank circuit whichoscillates at its oscillating frequency with the active element such asa transistor. Magnetic flux formed by the sensor coil 129 extendsthrough the conductive film on the substrate 124 placed in front of thesensor coil 129 and alternately changes to generate an eddy current inthe conductive film. Then, this eddy current flows through theconductive film to produce an eddy current loss, causing a reduction ina reactance component of the impedance of the sensor coil 129 from aview point of equivalent circuit.

[0032] Thus, when the eddy current loss is zero, the oscillator circuitoscillates at the oscillating frequency of the tank circuit. However, asthe eddy current loss exists, a resistance component in the oscillatorcircuit is increased due to the influence of an equivalent resistancecomponent of a semiconductor wafer, thereby causing the oscillatingfrequency to move toward a higher region. Therefore, by observing achange in the oscillating frequency of the oscillator circuit, it can beseen that as a conductive film is gradually removed with the advancementof the polishing, the oscillating frequency is correspondingly reduced,and at the time the conductive film is completely removed by thepolishing, the oscillating frequency becomes equal to the selfoscillating frequency of the tank circuit, followed by a substantiallyuniform oscillating frequency. In this way, it is possible to detect anend point in the chemical mechanical polishing of a conductive film bydetecting the point at which the oscillating frequency becomes equal tothe self oscillating frequency of the tank circuit.

[0033] According to the end point detection for the chemical mechanicalpolishing process utilizing the eddy current sensor, the advancement ofpolishing performed on a conductive film can be known during thepolishing without contacting a substrate subjected to the polishing.

[0034] The foregoing apparatus has the sensor coil 129 disposed in thetop ring 123 for holding a substrate subjected to polishing, and theactive element unit 130 of the oscillator circuit disposed in a fixturefor holding the top ring shaft 126 spaced away from the sensor coil 129,wherein the two components are connected through the communication line131. An oscillating signal formed by the oscillator circuit isintroduced into a personal computer through the interface box 133,distribution box 134 and the like by a communication line 136, so that atransition of the oscillating frequency is displayed on a monitor screenof the computer. The communication line 136 is comprised of a total offour wires which include a pair of signal lines and a pair of DC powerlines. Also, the communication line 131 is connected to the sensor coil129 which is contained in the top ring for rotating the fixed activeelement unit 130 using a rotary connector. Thus, as the polishing of aconductive film advances, an eddy current loss decreases, thereby makingit possible to observe how the oscillating frequency changes on thescreen of the monitor 135 of the personal computer 132.

[0035] The conventional method of detecting a polishing end point basedon the eddy current sensor, however, suffers from the followingproblems. Specifically, since the sensor coil 129 and active elementunit 130 are placed at separate positions and are interconnected by thehigh impedance communication line 131 through the rotary connector, thecommunication line 131 picks up noise associated with the rotation ofthe turn table and the like. The removal of the noise is difficult inthe course of output signal processing in the oscillator circuit. Forthis reason, a filter circuit or the like is required for attenuating asufficient amount of noise. The communication line 136 also picks upnoise.

[0036] Further, the oscillating frequency used for the eddy currentsensor is relatively low, i.e., approximately 7 MHz, so that a largeeddy current loss can be detected when a conductive film subjected topolishing has a sufficient thickness, whereas when the eddy current lossbecomes smaller as the conductive film is polished more so that itsthickness becomes extremely smaller, in which case difficulties areencountered in detecting, for example, a thickness of approximately 1000Å or less. In other words, because of a relatively low oscillatingfrequency utilized for detection, the conventional eddy current sensorfails to provide a sufficiently high accuracy for detecting an end pointfor polishing performed by a polishing apparatus which requires athickness detection accuracy on the order of angstroms.

SUMMARY OF THE INVENTION

[0037] The present invention has been proposed to solve the problems inthe frequency measurement of the above-described conventional eddycurrent sensor, and, therefore, an object of the present invention is toprovide a frequency measuring device and method capable of obtaining afrequency measurement result with high precision and in a short timeinterval, and a polishing device and method using the same.

[0038] Further, the present invention has an object to provide an eddycurrent sensor capable of operating stably and detecting an end point ofthe polishing accurately.

[0039] In order to achieve the objects described above, according to afirst aspect of the present invention, there is provided a device formeasuring a frequency of a measured signal, the device comprising:

[0040] counting means including a plurality of n-nary counters; and

[0041] gate means for supplying the measured signal to an input of therespective n-nary counters in the order of given time intervals;

[0042] wherein a frequency measurement result of the measured signal issupplied from the counting means every given time interval.

[0043] According to a second aspect of the present invention, there isprovided a method for measuring the frequency of a measured signal, themethod comprising:

[0044] providing counting means including a plurality of n-narycounters; and

[0045] supplying the measured signal to an input of the respectiven-nary counters in the order of given time intervals;

[0046] wherein a frequency measurement result of the measured signal issupplied from the counting means every given time interval.

[0047] According to a third aspect of the present invention, there isprovided a device for measuring the frequency of a measured signal,comprising:

[0048] a counting section including a number i(i≦2) of n-nary counters;

[0049] a time reference circuit that outputs a time reference signal, aduration of which is t, every time interval p; and

[0050] a number i of gate circuits where the respective outputs of whichare connected to the inputs of the n-nary counters, each of the gatecircuits having a first input that receives the measured signal, and asecond input that receives the time reference signal at the timeintervals p;

[0051] wherein the frequency measurement result of the measured signalis supplied from the counting section every time interval p.

[0052] It is preferable that t=i·p.

[0053] According to a fourth aspect of the present invention, there isprovided a polishing device comprising:

[0054] a turn table having a polishing surface;

[0055] a top ring for holding an object to be polished; and

[0056] an end point detecting mechanism for informing an end point ofthe polishing;

[0057] wherein the end point detecting mechanism includes a frequencymeasuring device that comprises:

[0058] counting means including a plurality of n-nary counters; and

[0059] gate means for supplying the measured signal to an input of therespective n-nary counters sequentially at given time intervals;

[0060] wherein a frequency measurement result of the measured signal issupplied from the counting means every given time interval.

[0061] According to a fifth aspect of the present invention, there isprovided a polishing method of informing of an end point in polishing ofan object by a turn table having a polishing surface, the methodcomprising the steps of:

[0062] providing counting means including a plurality of n-narycounters; and

[0063] supplying the measured signal to an input of the respectiven-nary counters in the order of given time intervals;

[0064] wherein a frequency measurement result of the measured signal issupplied from the counting means every given time interval.

[0065] Since the frequency measuring device according to the presentinvention has the above-mentioned structure, the present invention canprovide the frequency measured result of the measured signal every giventime interval at which an arbitrary length can be set, thereby resultingin such an advantage that the measured result can be obtained everyperiod shorter than that in the conventional frequency measuring device.In addition, an outstanding effect of increasing significant digits ofthe measured frequency can be obtained. Therefore, by applying such afrequency measuring device to the polishing device, a precision withwhich the end point of polishing of the semiconductor wafer is detectedcan be remarkably improved.

[0066] According to a sixth aspect of the present invention, there isprovided an eddy current sensor for detecting the thickness of anelectrically conductive film from a change in an eddy current lossgenerated in the conductive film, comprising:

[0067] a sensor coil for generating an eddy current in the conductivefilm; and

[0068] an active element unit connected to and integrally formed withthe sensor coil for oscillating a variable frequency corresponding tothe eddy current loss. The sensor coil is preferably in the shape of ahollow spiral, and a substrate for mounting the active element unit ispreferably disposed perpendicular to the hollow spiral sensor coil. Alsopreferably, an oscillator circuit integrally formed of the sensor coiland active element unit is connected to a low impedance coaxial cablewhich may serve as a power supply line and as an oscillating signaloutput line.

[0069] As described above, since the oscillator circuit is integrallyformed of the sensor coil forming part of the eddy current sensor, andthe active element unit connected to the sensor coil, the coaxial cablecan communicate signals at a low impedance (50 Ω), and the oscillatorcircuit can be stably operated without picking up noise caused byrotation of the turn table and the like of an associated polishingapparatus. Also, the substrate including the active element unit isdisposed in a direction perpendicular to the sensor coil, so that aneddy current loss can be detected using a high oscillating frequency inthe VHF band. It is therefore possible to detect an eddy current loss ina thin conductive film having a high resistivity and to detect apolishing state of a thin film on the order of angstroms such as atantalum (Ta) film or the like which is used as a barrier layer. Forthis reason, a polishing end point can be detected in a significantlyhigh accuracy.

[0070] According to a seventh aspect of the present invention, there isprovided an eddy current sensor for detecting the thickness of aconductive film from a change in an eddy current loss generated in theconductive film, comprising a sensor coil for generating an eddy currentin the conductive film, wherein a change in the thickness of theconductive film is detected from a change in a resistance component inan impedance formed by the sensor coil and the conductive film. Sincethis eddy current sensor detects a change in the thickness of aconductive film associated with the advancement of polishing byobserving a change in a resistive component with the oscillatingfrequency left fixed, it is possible to clearly observe a polishingstate of an extremely thin film at a relatively low frequency. Thus, theeddy current sensor has the ability to readily detect the thickness ofan extremely thin barrier layer which has a low conductivity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0071] These and other objects and features of the present inventionwill become more apparent when reading the following description withreference to the accompanying drawings, in which:

[0072]FIG. 1 is a cross-sectional view schematically showing thestructure of a polishing device proposed by the applicant of the subjectapplication;

[0073]FIG. 2 is a plan view showing a positional relationship of a turntable, a semiconductor wafer and an eddy current sensor in the polishingdevice shown in FIG. 1;

[0074]FIGS. 3a and 3 b show enlarged main-portion cross-sectional viewsschematically showing a state in which the eddy current sensor isembedded in the polishing device shown in FIG. 1;

[0075]FIG. 4 is a graph showing a result of processing a detectionsignal of the eddy current sensor obtained during the polishing of asemiconductor wafer by the polishing device shown in FIG. 1;

[0076]FIGS. 5a and 5 b are a block diagram schematically showing thestructure of a conventional frequency measuring device.

[0077]FIG. 6 illustrates the main portion of a conventional polishingapparatus.

[0078]FIG. 7 is a block diagram schematically showing the structure of afirst embodiment of a frequency measuring device according to thepresent invention;

[0079]FIG. 8 is a block diagram schematically showing the structure of asecond embodiment of a frequency measuring device according to thepresent invention;

[0080]FIG. 9 is a perspective view schematically illustrating thestructure of an eddy current sensor according to the present invention;

[0081]FIGS. 10a and 10 b are graphs showing how an oscillating frequencychanges with the advancement of polishing;

[0082]FIG. 11 is a circuit block diagram illustrating a detector circuitfor detecting an oscillating signal of the eddy current sensor shown inFIG. 9;

[0083]FIG. 12 is a block diagram illustrating an amplitude adjustingcircuit in FIG. 11;

[0084]FIG. 13a is a graph showing a trajectory of a change in theoscillating frequency;

[0085]FIG. 13b is a graph showing a trajectory of a change in atime-differentiated value of the oscillating frequency;

[0086]FIG. 14 is a graph showing a trajectory of a change in anequivalent impedance as measured by an LCR meter;

[0087]FIG. 15 is a block diagram illustrating the structure of anotherembodiment of an eddy current sensor according to the present invention;

[0088]FIG. 16 shows a trajectory of changes in a resistance component(R) vs. a reactance component (X) associated with a change in thickness,as detected by the eddy current sensor illustrated in FIG. 15;

[0089]FIGS. 17a, 17 b and 17 c are graphs each showing an exemplarychange in the resistance component (R) and reactance component (X)caused by a change in thickness;

[0090]FIG. 18 is a vertical sectional view schematically illustrating anoverall structure of a polishing apparatus according to the presentinvention;

[0091]FIG. 19 is a top plan view of a turn table of the polishingapparatus illustrated in FIG. 18;

[0092]FIGS. 20a and 20 b are graphs showing exemplary results ofdetected oscillating signals in the polishing apparatus illustrated inFIG. 18;

[0093]FIG. 21 is a graph showing an exemplary calibration for theoscillating frequency of the eddy current sensor and the thickness; and

[0094]FIGS. 22a and 22 b are perspective views illustrating structuralexamples of another polishing apparatus according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0095] Now, a description will be given of preferred embodiments of afrequency measuring device using a sequential counting system, accordingto the present invention as described herein. First, referring to FIG.7, a frequency measuring device FC includes i de counters 1, 2, 3, 4, .. . i, in which i is an integer of 2 or more. The input terminals ofthese decimal counters 1-i are connected to the output terminals ofrespective corresponding gate circuits G1-Gi, and also the outputs ofthe decimal counters 1-i are connected to the input terminal of a latchcircuit 11 in parallel. The latch circuit 11 supplies a value latched bythe latch circuit 11 to a desired processing circuit through an I/O port12 at given intervals.

[0096] A measured signal Vin having a pulse waveform is supplied to aninput terminal IN of an amplifier A, and the measured signal Vinamplified by the amplifier A is supplied to one input terminal of eachof the gate circuits G1-Gi in parallel. The other input terminals ofthese gate circuits G1-Gi are supplied with a time reference signal Tsequentially outputted from a time reference circuit 13. In order todetermine the operation timing of the time reference circuit 13, a clocksignal outputted from a clock oscillating circuit 14 is supplied to thetime reference circuit 13.

[0097] Hereinafter, a description will be given of the frequencymeasurement of the signal to be measured by the frequency measuringdevice shown in FIG. 7. The time reference circuit 13 sequentiallyoutputs the reference time T, whose duration is t, at given timeintervals p (<t) and supplies the time reference signal T to the otherinput terminals of the gate circuits G1-Gi so that the respective gatecircuits G1-Gi are opened in a respective order according to a delay ofthe time interval p and for a period of the duration time t. Forexample, if the time reference circuit 13 supplies the time referencesignal T to the other input terminal of the gate circuit G1 to open thegate circuit G1, the time reference circuit 13 supplies the timereference signal T to the other input terminal of the gate circuit G2 toopen the gate circuit G2 after the time interval p from the time whenthe gate circuit G1 is opened, and also supplies the time referencesignal T to the other input terminal of the gate circuit G3 to open thegate circuit G3 after the time interval p from the time when the gatecircuit G2 is opened. Similarly, the time reference circuit 13 opens thegate circuits G4-Gi in the order of the time intervals p, and opens thegate circuit G1 after the time interval p from the time when the gatecircuit Gi is opened.

[0098] In this way, the gate circuits G1-Gi open in a respective orderaccording to the time intervals p and repeat the opening operation in aperiod of the duration t. As a result, the respective decimal counters1-i count the number of pulses of the measured signals Vin that havepassed through the corresponding gate circuits G1-Gi in the period ofthe duration time t where the respective corresponding gate circuitsG1-Gi are opened in accordance with the time reference signal T, andoutput the count values to the latch circuit 11 in order. In responsethereto, the latch circuit 11 outputs the latched value to an I/O port12 every time the latch circuit 11 receives the count value from any oneof the decimal counters 1-i. As a result, the frequency measuring deviceFC can supply the frequency measuring result of the measured signal Vinevery time interval p after the duration t elapses from the point whenthe operation has started.

[0099] In order that the frequency measuring device FC shown in FIG. 7may conduct the above-mentioned operation, it is necessary that t=i·pand the number i of the decimal counters is an arbitrary integer of 2 ormore. For example, when i=10, if t is 100 ms, p becomes 10 ms, and thefrequency measuring device FC can supply the measured result of thefrequency up to 10 MHz with the significant digits of 6 digits every 10ms after 100 ms elapses from the time when the operation has started.Also, assuming that i=2, t is 100 ms, and p is 50 ms, the frequencymeasuring device FC can supply the measured result of the frequency upto 10 MHz with the significant digits of 6 digits every 50 ms after 100ms elapses from the time when the operation has started, and assumingthat i=5, t is 100 ms, and p is 20 ms, the frequency measuring device FCcan supply the measured result of the frequency up to 10 MHz with thesignificant digits of 6 digits every 20 ms after 100 ms elapses from thetime when the operation has started.

[0100] In FIG. 7, decimal counters are used, but, the present inventionis not limited thereto, and arbitrary n-nary digital counters can beused. For example, in FIG. 8, which schematically shows a frequencymeasuring device in accordance with another embodiment of the presentinvention, i binary counters are used. That is, the frequency measuringdevice FC has 20-bit binary counters 21, 22, 23, 24, . . . , 2 i, andthe inputs of these binary counters 21-2 i are supplied with the outputsof the respective corresponding gate circuits G1-Gi. In this case, sincea binary measured signal is outputted from the latch circuit 11, abinary-to-decimal converting circuit 15 is disposed at the output sideof the latch circuit 11, and the binary measured result is convertedinto the decimal measured result.

[0101] In the operation of the frequency measuring device FC shown inFIG. 8, the operation of the gate circuits G1-Gi, the time referencecircuit 13, the I/O port 12 and the clock oscillating circuit 14 areidentical with that in the first embodiment shown in FIG. 7 except thatthe binary counters 21-2 i output the frequency measured result inbinary form and the latch circuit latches the binary number, andtherefore a description thereof will be omitted here.

[0102] As described above, since the frequency measuring device shown inFIGS. 7 and 8 can supply a frequency measured result at intervalsshorter than that of the conventional device, such a frequency measuringdevice can be applied to the end point detecting mechanism of thepolishing device shown in FIG. 1, thereby making it possible to detectthe end point of polishing with higher precision.

[0103]FIG. 9 schematically illustrates the structure of an eddy currentsensor according to an embodiment of the present invention. Theillustrated eddy current sensor 30 comprises a sensor coil 31 forgenerating an eddy current in a conductive film, and an active elementunit 32 integrated with and connected to the sensor coil 31 to form anoscillator circuit for generating a variable frequency signalcorresponding to an eddy current loss. The sensor coil 31 and asubstrate (not shown) mounting the active element unit 32 thereon areaccommodated in a box 33. The box 33 has a length and a width ofapproximately 20 mm or less, and a height of approximately 10 mm orless. A coaxial cable 34 having an impedance of approximately 50 (isconnected to the active element unit 32 for supplying the eddy currentsensor 30 with DC power. The cable 34 also serves as an output line forrouting out an oscillating signal.

[0104] The sensor coil 31 is in the shape of a hollow spiral having twoturns in this embodiment. The substrate mounting thereon the activeelement unit 32, which comprises the oscillator circuit, is positionedperpendicular to the hollow spiral sensor coil 31, so that the sensorcoil 31 does not generate an eddy current in a conductive material onthe substrate which includes the active element unit 32. Specifically,if the sensor coil 31 was positioned in parallel with the substratemounting the active element unit 32 thereon, magnetic flux generatedfrom the sensor coil 31 would induce an eddy current in the conductivematerial on the circuit substrate. Then, the eddy current sensor woulddetect this eddy current loss, thereby resulting in a degraded accuracy.In addition, for the substrate which mounts the active element unit 32thereon, the eddy current generated in the conductive film on thesubstrate is not preferable for its operation. Therefore, disposing thesensor coil 31 perpendicular to the substrate including the activeelement unit 32 enables the resulting eddy current sensor to accuratelymeasure an eddy current loss at a high oscillating frequency, forexample, on the order of 200 MHz, as will be described later in greaterdetail.

[0105] A Colpitts-type oscillator circuit, for example, may be employedfor the active element unit 32, wherein a tank circuit is formed fromthe inductance of the sensor coil 31 and the capacitance of a capacitormounted on the substrate. As described above, the oscillating frequencyof the eddy current sensor is determined by the oscillating frequency ofthe tank circuit. Also as described above, an eddy current loss causes acorresponding change in a reactance component with equivalent impedanceof the sensor coil 31 to shift the oscillating frequency.

[0106] In the embodiment shown in FIG. 9, the oscillating frequency isset to approximately 200 MHz in a VHF band by appropriately selectingthe inductance of the sensor coil 31 and the capacitance of thecapacitor mounted on the active element unit 32. The selection of suchan oscillating frequency results in a detection sensitivitycorresponding to the resistivity of a conductive layer which generatesan eddy current loss. Specifically, an electrically conductive filmpossibly subjected to chemical mechanical polishing may generallycomprise a barrier layer made of tantalum (Ta) film, and a plating layermade of copper (Cu) deposited on the barrier film. The tantalum (Ta)film has the resistivity of approximately 160 Ω, while the copper (Cu)has the resistivity of approximately 1.6 Ω, from which a difference ofapproximately 100 times is appreciated between the two materials. Whenthe conductive film is made of copper (Cu), a resulting oscillatingfrequency is detected at approximately 20 MHz, a good accuracy, as shownin FIG. 10a. Specifically, with the copper film having a sufficientthickness, the oscillating frequency is detected around 20.7 MHz. Whenmost of the copper (Cu) film has been removed, the oscillating frequencyis detected around 20.0 MHz. Thus, a sufficient detection window isprovided over approximately 0.7 MHz between a conductive film having asufficient thickness and the conductive film being substantiallyremoved. For a tantalum (Ta) film used as a barrier layer, anoscillating frequency is generated at approximately 187 MHz when thetantalum (Ta) film has a sufficient thickness, and at 184 MH when thetantalum (Ta) film has been substantially removed. As is the case withthe copper film, a sufficient detection window is provided on the orderof 3 MHz.

[0107] The tantalum (Ta) film, serving as a barrier layer, has athickness on the order of angstroms, while the copper (Cu) film has athickness on the order of micrometers (Jim). Therefore, the eddy currentsensor 30 illustrated in FIG. 9 is capable of detecting the advancementof polishing an extremely thin tantalum (Ta) film which forms a barrierlayer. Specifically, for detecting the advancement of polishing a copperfilm using the oscillating frequency at 7 MHz, the end point of thepolishing is detected, for example, with an error of approximately 1000Å, whereas the end point for polishing of the extremely thin tantalum(Ta) film or the barrier layer can be detected on the order of angstromsusing the oscillating frequency set at approximately 180 MHz. In thisway, the accuracy can be significantly improved for detecting the endpoint of the polishing.

[0108]FIG. 11 illustrates a detector circuit using the eddy currentsensor 30 for detecting an eddy current loss. As described above, theeddy current sensor 30 comprises the sensor coil 31, capacitors 41, 42which form a tank circuit together with the sensor coil 31, and anactive circuit element 43 such as a transistor or the like. Thecapacitance includes a fixed capacitor 41 and a variable capacitor 42.The variable capacitance 42 forms part of an automatic frequency tuningcircuit, as later described in detail. A divider or a subtractor 44 anda distribution board 134 (FIG. 6) for performing a waveform conversionare connected to the eddy current sensor 30 through a coaxial cable 34.As described above, the coaxial cable 34 serves as a power supply lineas well as a signal line, so that an oscillating signal from the eddycurrent sensor 30 is fed to the oscillating signal detector circuitthrough a coupling capacitor and DC power is supplied from the interfaceboard 133. The divider is provided for the purpose of dividing thedetected oscillating frequency, and its resolution can be increased byremoving a majority of fixed components with respect to a variablecomponent through a subtractive operation of the subtractor.

[0109] An oscillating signal detecting circuit in a processor 132 relieson a change in the eddy current loss associated with the advancement ofpolishing to detect how the polishing is advanced. Generally, twopossible methods may be employed for this purpose. A first methodinvolves detecting a change in the oscillating frequency of theoscillating signal. As shown in FIGS. 10a and 10 b, as a conductive filmis increasingly polished, the eddy current loss also changes and anequivalent resistance value of the sensor coil changes. This results ina change in the oscillating frequency of the oscillator circuit, so thata signal corresponding to the frequency within the detection window isdisplayed on a monitor by dividing the oscillating signal using thedivider or by subtracting the same using the subtractor. Consequently,the graphs as shown in FIGS. 10a and 10 b are drawn for indicating thetrajectories of the changing frequencies.

[0110] When a conductive film subjected to polishing has a sufficientthickness, a small change is given in the eddy current loss associatedwith the advancement of the polishing (over time t), thus resulting in asmall change in the oscillating frequency. As the polishing is advancedto reduce the thickness of the conductive film, the eddy current losssuddenly decreases. This causes a sudden decrease in the oscillatingfrequency as well. Eventually, when the residual conductive film iscompletely removed, the oscillating frequency remains substantiallyconstant because of the absence of the conductive film even though anunderlying oxide film is subsequently polished. Thus, the end point isdefined at a point at which the oscillating frequency substantiallysettles after its sudden decrease. The output of the eddy current sensorundergoes a moving average calculation and then differentiation. Thepolishing end point can be accurately detected by observing the resultof differentiation.

[0111] By communicating the oscillating frequency detected by theoscillating frequency detecting unit to a controller circuit and bychanging the capacitance of the variable capacitor (varicap) 42, a shiftin the oscillating frequency is automatically corrected throughautomatic frequency tuning. The automatic frequency tuning can suppressfluctuations in the self oscillating frequency of the sensor foreliminating an individual difference of the sensor to stabilize thesensitivity for the output signal frequency from the eddy currentsensor, thereby eliminating variations in the eddy current sensor itselfdue to the manufacturing accuracy. For stabilizing the oscillatingamplitude of the oscillator circuit using an automatic amplitude control(ALC) method to provide a constant amplitude, a high frequency amplitudedetector 45 may be provided in the oscillating signal detector circuitfor detecting a signal, the magnitude of which is compared with areference amplitude signal in a comparator 46 which manipulates anattenuator 47 to control the amplitude constant, as illustrated in FIG.12. The introduction of such a circuit provides for a stabilizedoperation for a conversion of the high frequency signal from the eddycurrent sensor to a TTL level signal as well as for a stable S/N ratio.

[0112] The oscillating frequency signal from the eddy current sensor maybe regarded as a temporal gradient change of frequency, in other words,the oscillating frequency may be differentiated with respect to thetime, and the polishing end point can be determined from characteristicpoints on a resulting curve which represents the differentiatedoscillating frequency over time. FIG. 13a shows a trajectory of a changein the oscillating frequency itself over time t, and FIG. 13b shows atrajectory of the differentiated oscillating frequency. In these graphs,A indicates a time point at which a metal layer is removed; B indicatesa time point at which a first barrier layer is removed; and C indicatesa time point at which a second barrier layer is removed. Even though theoscillating frequency itself presents a slight change, thedifferentiated values may be observed to readily detect the end pointfor the polishing of a barrier layer in the order of angstroms.

[0113] A second eddy current loss detecting method involves directlymeasuring a resistance component in equivalent impedance of the eddycurrent loss caused by the sensor coil 31 using an LCR meter. The LCRmeter may be used as the signal detector circuit in FIG. 11 to displaythe resistance R on the horizontal axis and the reactance X on thevertical axis on a monitor screen as illustrated in FIG. 14. From thefact that an eddy current loss in a conductive film changes with theadvancement of polishing, it is possible to observe how the resistance Rand reactance X take a trajectory associated with the change in the eddycurrent loss in the conductive film. Specifically, at point B, a largeamount of conductive film still remains so that a large eddy currentloss is generated, while at point A, the conductive film is polished tocomplete removal thereof, with the resulting elimination of the eddycurrent loss, so that a fixed resistance component remains alone whenviewed from the impedance meter. As shown in FIG. 14, a change inimpedance of the sensor is expressed by:

ΔR>>ΔX

[0114] As noted, the reactance component (ΔR) presents a significantlylarger change than the resistance component (ΔX) does. If a result ofmeasurement deviates from a predetermined range when the polishingapparatus is being operated using the eddy current sensor, the sensor isdetermined to fail, and an error signal is generated. Then, thepolishing can be interrupted to minimize the influence upon failure.

[0115]FIG. 15 illustrates another embodiment of an eddy current sensoraccording to the present invention. A sensor coil 51 of an eddy currentsensor 50 is in the shape of hollow spiral, similar to that of the eddycurrent sensor 30 in the first embodiment. The actually employed coil 51has, for example, two turns. The sensor coil 51 is positioned near asemiconductor wafer W having a conductive film, subjected to polishing,deposited thereon. A signal source for supplying the sensor coil 51 withan AC signal is an oscillator unit 52 including a quartz oscillator at afixed frequency, which generates signals at fixed frequencies of 8, 16,32 MHz, by way of example. A voltage detected across the sensor coil 51passes through a bandpass filter 53 which passes therethrough anoscillating frequency of the oscillator unit 52. Then, a cosine (cos)component and a sine (sin) component are extracted from the detectedsignal by a synchronous detector unit which includes a cos synchronousdetector circuit 54 and a sin synchronous detector circuit 55. A phaseshifter circuit 56 generates two signals, i.e., an in-phase component(0°) and a quadrature component (90°) from the oscillating signalgenerated by the oscillator unit 52. The two components are introducedinto the cos synchronous detector circuit 54 and sin synchronousdetector circuit 55, respectively, for performing synchronous detection,as described above.

[0116] Synchronously detected signals have their unnecessary highfrequency components above signal components removed by low pass filters57, 58, respectively, to extract a resistance component (R) output whichis the cos synchronous detection output, and a reactance component (X)output which is the sin synchronous detection output. Also, a vectorcalculating circuit 59 calculates an amplitude output (R²+X²)^(1/2) fromthe R component output and X component output. Similarly, a vectorcalculating circuit 60 calculates a phase output (tan⁻¹·R/X) from the Rcomponent output and X component output.

[0117]FIG. 16 shows an exemplary result of measuring the thickness of aconductive film using the eddy current sensor 50, where the horizontalaxis represents the resistance component (R), and the vertical axisrepresents the reactance component (X). A point A is observed when thethickness is extremely large, for example, 100 μm or more. At the pointA, the impedance viewed from the sensor coil presents an extremely smallresistance component (R) viewed from the sensor coil since an extremelylarge eddy current loss is generated in the conductive film placed nearthe sensor coil. As the polishing is advanced so that the conductivefilm becomes thinner, the resistance component (R) as well as thereactance component increase as viewed from the sensor coil. The pointat which the resistance component (R), viewed from the sensor coil,reaches its maximum is indicated by B. As the polishing is furtheradvanced to thin the conductive film, the resistance (R) componentviewed from the sensor coil becomes gradually smaller as the eddycurrent loss is gradually reduced. Eventually, when the conductive filmis completely removed by the polishing, no eddy current loss exists sothat the resistance component (R) caused by the eddy current lossbecomes zero, leaving only the resistance of the sensor coil itself Thereactance component (X) at this time shows the reactance component ofthe sensor coil itself This state is indicated by a point C.

[0118] Actually, for depositing a copper wire in a groove formed in asilicon oxide film by a so-called Damascene process, a barrier layermade of tantalum nitride (TaN), titanium nitride (TiN) or the like isdeposited on the silicon oxide film, and a metal wire made of highlyconductive copper, tungsten or the like is deposed on the barrier layer.Therefore, the detection of an end point for polishing the barrier layeris critical in polishing the conductive film. However, as mentionedabove, the barrier layer employed herein is made of a film having arelatively low conductivity such as tantalum nitride (TaN), titaniumnitride (TiN) or the like in an extremely small thickness on the orderof angstroms.

[0119] The eddy current sensor according to the second embodiment of thepresent invention can readily detect the thickness of a barrier layer asmentioned above near the polishing end point. Specifically, as shown inFIG. 16, a point D indicates a position at which the thickness isapproximately 1000 Å. From the point D to the point C at which thethickness is zero, the resistance component exhibits an extremely largeand substantially linear change in correspondence with a reduction inthe thickness. In this event, the reactance component (X) exhibits anextremely small amount of change as compared with the resistancecomponent, as can be seen from FIG. 16. For this reason, theaforementioned eddy current sensor 30 according to the first embodiment,which relies on the basic idea that the thickness is detected based on achange in the oscillating frequency caused by a change in the reactancecomponent, merely detects an extremely small change in the oscillatingfrequency with respect to the change in the thickness. For increasingthe resolution for the frequency change to overcome this disadvantage,the oscillating frequency must be increased as described above. Incontrast, the eddy current sensor 50 according to the second embodimentrelies on a change in a resistance component to detect a change inthickness with the oscillating frequency left unchanged, thereby makingit possible to clearly observe how an extremely thin film is polished ata relatively low frequency.

[0120]FIGS. 17a, 17 b and 17 c show results of detecting the thicknessof an extremely thin conductive film on the order of angstroms, wherethe horizontal axis represents a remaining thickness; the left-handvertical axis represents a resistance component (R); and the right-handvertical axis represents a reactance component (X). FIG. 17a shows dataon a tungsten (W) film, from which it can be seen that a change inthickness can be clearly detected by observing a change in resistancewith the remaining film having an extremely small thickness of 1000 Å orless. FIG. 17b shows data on a titanium nitride (TiN) film, from whichit can be similarly seen that a change in thickness can be clearlydetected in a region of 1000 Å or less. FIG. 17c shows data on atitanium (Ti) film, in which a change in thickness can be clearlydetected since the resistance component changes from approximately 100 Ωor more to approximately 0 Ω while the thickness is reduced from 500 to0 Å, as can be seen in FIG. 1C.

[0121] Preferably, the signal source generates a higher oscillatingfrequency, for example, approximately 32 MHz for detecting a barrierlayer which has a relatively low conductivity. A higher oscillatingfrequency allows for a clear observation of a change in the thickness ofthe barrier layer from 0 to 250 Å. On the other hand, with a metal filmsuch as a copper film, a tungsten film or the like having a relativelyhigh conductivity, a change in thickness can be clearly detected with alow oscillating frequency. In this way, the oscillating frequency ispreferably selected depending on the type of film subjected to thepolishing.

[0122] In each of the examples shown in FIGS. 17a-17 c, the reactancecomponent (X) presents an extremely small change with respect to achange in the resistance component (R). In the examples of detecting thethickness of the barrier layer, a change in the reactance component (X)is 0.005% with a tantalum film having a remaining thickness of 0 Å and250 Å, whereas a change in the resistance component (R) is 1.8%. It isappreciated that the detection sensitivity is improved by a factor ofapproximately 360 over the conventional method which relies on a changein the reactance component.

[0123] The foregoing exemplary method of detecting a film thicknessmainly relies on a change in the resistance component (R). However, theeddy current sensor 50 illustrated in FIG. 15 can extract an amplitudeoutput and a phase output associated with the advancement of polishing.Therefore, these signal outputs may be used to check for the advancementof polishing in multiple aspects, for example, by measuring thethickness based on a detected phase angle.

[0124]FIG. 18 is a vertical sectional view generally illustrating theconfiguration of a polishing apparatus which employs the foregoing eddycurrent sensor 30 or 50. As illustrated in FIG. 18, the polishingapparatus comprises a turn table 71, and a top ring (holder) 72 forholding and pressing a semiconductor wafer 73 onto an abrasive cloth 74on the turn table 71. The turn table 71 is coupled to a motor 75 to berotatable about the center axis thereof, as indicated by an arrow. Thetop ring 72 is coupled to a motor (not shown) as well as to an elevatingcylinder (not shown). In this structure, the top ring 72 can be moved upand down and rotated about the center axis as indicated by arrows, suchthat the semiconductor wafer 73 can be pressed onto the abrasive cloth74 with an arbitrary force. The top ring 72 is coupled to a top ringshaft 76, and also includes an elastic mat 77 made of polyurethane orthe like on the bottom. The top ring 72 is also provided with a guidering 78 around the outer periphery of a lower portion of the top ring 72for preventing the semiconductor wafer 73 from coming off. A polishingsolution nozzle 79 is disposed above the turn table 71 for supplying apolishing solution Q onto the abrasive cloth 74 adhered on the turntable 71.

[0125] As illustrated in FIG. 18, an eddy current sensor 30 or 50 isembedded in the turn table 71. The eddy current sensor 30 or 50 isconnected to a controller 80 by a connection cable 34 (FIG. 9) which isrouted through the inside of the turn table 71 and turn table supportingshaft 71 a, and through a rotary connector (or a slip ring) 81 disposedat an end of the turn table supporting shaft 71 a. The controller 80 isconnected to a display unit 82.

[0126]FIG. 19 is a top plan view of the polishing apparatus illustratedin FIG. 18. As illustrated in FIG. 19, six eddy current sensors 30 a-30f (50 a-50 f) are embedded at positions which pass the center Cw of thesemiconductor wafer 73 held by the top ring 72 for the polishing. SymbolCT indicates the center of rotation of the turn table 71. The eddycurrent sensors 30 a-30 f(50 a-50 f) are configured to sequentiallydetect the thickness of a conductive film such as a Cu layer, a barrierlayer or the like on the semiconductor wafer 73 on the trajectory whilethey are passing below the semiconductor wafer 73. In this event, theeddy current sensors may use a plurality of different frequencies fordetecting the thickness. In this way, the detection can be managed suchthat a change in the thickness of a barrier layer is mainly detected athigher frequencies, while a change in the thickness of a conductivelayer is mainly detected at lower frequencies.

[0127] While the eddy current sensors are embedded at six locations inthe example illustrated in FIG. 19, the number of eddy current sensorsmay be changed as appropriate. Also, while the foregoing embodiment hasbeen described for a polishing apparatus which has an abrasive cloth 74adhered on the turn table 71, a fixed abrasive grain plate may be usedinstead. In this case, the eddy current sensors may be disposed withinthe fixed abrasive grain plate. In addition, a polishing apparatus mayhave a plurality of turn tables 71 instead of one. Alternatively, theturn table 71 may be replaced with a belt or a web which has an abrasivesurface.

[0128] In the polishing apparatus configured as described above, thesemiconductor wafer 73 is held on the bottom of the top ring 72 andpressed against the abrasive cloth 74 on the top of the turn table 71 bythe elevating cylinder. By supplying the polishing solution Q from thepolishing solution nozzle 79, the polishing solution Q is retained onthe abrasive cloth 74, so that the semiconductor wafer 73 is polishedwith the polishing solution Q intervening between a polished surface(lower side) of the semiconductor wafer 73 and the abrasive cloth 74.

[0129] During the polishing, the eddy current sensors 30 a-30 f(50 a-50f) pass immediately beneath the polished surface of the semiconductorwafer 73, respectively, each time the turn table 71 makes one rotation.In this event, since the eddy current sensors 30 a-30 f(50 a-50 f) arepositioned on the trajectory which passes the center Cw of thesemiconductor wafer 73, the thickness of the semiconductor wafer 73 canbe sequentially detected on the arcuate trajectory of the polishedsurface of the semiconductor wafer 73 as the sensors are moved. In thisevent, since the eddy current sensors 30 a-30 f (50 a-50 f) are mountedat six locations, the advancement of polishing can be detected by any ofthe sensors at short intervals, though intermittently

[0130] In the end point detecting method using the conventional eddycurrent sensor illustrated in FIG. 6, since the eddy current sensor coil129 is disposed within the top ring 123, this method is disadvantageousin that the thickness of a conductive film such as a Cu layer formed ona semiconductor wafer 124 can be measured only beneath the eddy currentsensor. In this event, when an increased number of sensors are embeddedin the top ring, the thickness can be measured at the increased numberof locations. However, even with the provision of more eddy currentsensors, this method can only provide intermittently measured values ata plurality of mutually discrete points (or a large number of points)but is incapable of providing measured values as a continuous profile.In addition, the increased number of sensors disadvantageously requiresan increased manufacturing cost and complicated signal processing. Tothe contrary, in the polishing apparatus according to the presentinvention in which the eddy current sensor 30 or 50 is embedded in theturn table 71, the eddy current sensor passes beneath the polishedsurface of the semiconductor substrate 73 while the turn table 71 makesone rotation. In this event, since the eddy current sensor 30 or 50 ispositioned on the trajectory passed by the center of the semiconductorsubstrate 73, the thickness of the semiconductor substrate 73 can besequentially detected on the arcuate trajectory on the polished surfaceof the semiconductor substrate 73 as the sensors are moved.

[0131] As shown in FIGS. 20a, 20 b, values of signals from the eddycurrent sensor 30 or 50, processed by the controller 80, graduallydecrease as the polishing is advanced. Stated another way, as thethickness of a conductive film is reduced, a detected value obtained byprocessing, by the controller 80, the signals from the eddy currentsensor 30 or 50 becomes smaller. Therefore, an end point for a CMPprocess can be detected by monitoring the detected value output from thecontroller 12 with previous knowledge of the detected value which wouldbe outputted when the conductive film is removed except for wires.

[0132]FIG. 21 shows an example of a calibrated relationship between thethickness of the polished semiconductor wafer 73 and oscillatingfrequency. Assume, for example, that a reference wafer having athickness of 1000 Å (t1) or 200 Å (t₂) is prepared, and a frequency suchas an oscillating frequency f₁ or f₀ in the reference wafer is measuredand defined as a reference point. Then, data corresponding to a changein thickness is obtained for a change in frequency associated with theadvancement of actual polishing. Such data is indicated by a dottedline. This approach can be applied as well to a detection of aresistance component output. A curve is drawn for the obtained data withrespect to the reference point by an appropriate approach such as aleast square method or the like. A change in thickness can be directlyread from a change in the detected output when the characteristics ofthe eddy current sensor have been previously calibrated by such anapproach.

[0133] The polishing apparatus comprising a number of such eddy currentsensors can detect a polishing end point over the entirety of asemiconductor wafer, and moreover at short time intervals. In addition,since the polishing apparatus can detect an end point of the polishingof a barrier layer such as Ta, TaN, TiN layers as described above, ahighly accurate detection can be accomplished for the polishing endpoint.

[0134] Alternatively, the removal of a conductive film except for wiresmay be detected by processing signals from eddy current sensors and anoptical sensor and monitoring the processed signals to determine an endpoint for a CMP process. FIGS. 22a, 22 b illustrate exemplaryconfigurations of such a polishing apparatus. A belt-like polishing pad91 is driven by rollers 92, 93 to rotate, so that a polished material(semiconductor wafer) held by a top ring (holder) 95 is pressed onto thepolishing pad 91 as it is rotated. As a polished surface of the materialunder polishing is in sliding contact with the polishing pad 91(polishing surface), the polishing is advanced accordingly. A supporter96 mounted with the eddy current sensor and optical sensor is disposedbeneath the top ring for monitoring a surface state of the polishedsurface. A hole 97 (FIG. 22a) and a groove 98 (FIG. 22b) are providedfor the optical sensor to observe the surface state of the polishedsurface.

[0135] While the foregoing embodiment has been described for a Cu layerand a Ta layer as conductive films, the present invention can of coursebe applied to any other conductive film made of a metal such as Cr, W,Ti, and the like. In addition, the performance of the polishingapparatus for detecting a polishing end point can be improved by using(1) an eddy current sensor signal, (2) a current signal of the turntable motor or the top ring motor, (3) an optical signal of an opticalmeans disposed in the turn table or out of the turn table, incident toor reflected from the polished surface, and (4) a signal indicative ofthe temperature on the polished surface, alone or in any appropriatecombination.

[0136] As described above, according to the present invention, an endpoint of the polishing operation can be stably and accurately detectedin a polishing apparatus.

[0137] The foregoing description of the preferred embodiments of theinvention has been presented for purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed, and modifications andvariations are possible in light of the above teachings or may beacquired from practice of the invention. The embodiments were chosen anddescribed in order to explain the principles of the invention and itspractical application to enable one skilled in the art to utilize theinvention in various embodiments and with various modifications as aresuited to the particular use contemplated. It is intended that the scopeof the invention be defined by the claims appended hereto, and theirequivalents.

What is claimed is:
 1. A method of detecting the thickness of a film formed on a substrate, said method comprising the steps of: supplying a sensor coil with an alternating current for generating an eddy current in said film; measuring an impedance across said sensor coil; detecting the thickness of said film based on a resistance component in said impedance.
 2. The method according to claim 1, further comprising the step of dividing said impedance into said resistance component and a reactance component.
 3. The method according to claim 2, wherein said alternating current has a constant frequency.
 4. The method according to claim 2, wherein the frequency of said alternating current is changeable according to a type of said film.
 5. The method according to claim 2, further comprising the step of calculating an amplitude of the output of said sensor coil from said resistance component and said reactance component.
 6. The method according to claim 2, further comprising the step of calculating the phase of the output of said sensor coil from said resistance component and said reactance component.
 7. A method of detecting the thickness of a film formed on a substrate, said method comprising the steps of: supplying a sensor coil with an alternating current for generating an eddy current in said film; measuring an impedance across said sensor coil; detecting the thickness of said film based on a resistance component in said impedance; and dividing said impedance into said resistance component and a reactance component.
 8. The method according to claim 7, wherein said alternating current has a constant frequency.
 9. The method according to claim 8, wherein the frequency of said alternating current is changeable according to a type of said film.
 10. The method according to claim 8, further comprising the step of calculating the amplitude of said alternating current from said resistance component and said reactance component.
 11. The method according to claim 8, further comprising the step of calculating the phase of said alternating current from said resistance component and said reactance component.
 12. A method of detecting the thickness of a film by an eddy current sensor coil based on a change in a resistance component in an impedance formed by said sensor coil and said film.
 13. An eddy current sensor for detecting the thickness of a film from a change in an eddy current generated in said film, said sensor comprising: a sensor coil for generating an eddy current in said film; wherein said sensor coil detects a change in the thickness of said film from a change in a resistance component in an impedance formed by said sensor coil and said film.
 14. An eddy current sensor according to claim 13, wherein said film is a conductive film.
 15. An apparatus for polishing a film formed on a substrate, comprising an eddy current sensor for measuring the thickness of said film, said sensor being configured to detect the thickness of said film based on a resistance component in an impedance formed by said sensor and said film.
 16. An apparatus according to claim 15, wherein said sensor comprises a sensor coil.
 17. An apparatus according to claim 16, further comprising a synchronous detector for extracting said resistance component and a reactance component from said impedance.
 18. An apparatus according to claim 17, further comprising at least one vector calculating circuit for calculating an amplitude of the output of said sensor coil from said resistance component and said reactance component.
 19. An apparatus according to claim 17, further comprising at least one vector calculating circuit for calculating a phase of the output of said sensor coil from said resistance component and said reactance component.
 20. An apparatus according to claim 16, wherein said film is a conductive film.
 21. An apparatus for polishing a film formed on a substrate, comprising an eddy current sensor for measuring the thickness of said film, said sensor being configured to detect a change in the thickness of said film based on a change in a resistance component in an impedance formed by said sensor and said film.
 22. An apparatus according to claim 21, wherein said sensor comprises a sensor coil.
 23. An apparatus according to claim 22, further comprising a synchronous detector for extracting said resistance component and a reactance component from said impedance.
 24. An apparatus according to claim 23, further comprising at least one vector calculating circuit for calculating an amplitude of the output of said sensor coil from said resistance component and said reactance component.
 25. An apparatus according to claim 23, further comprising at least one vector calculating circuit for calculating a phase of the output of said sensor coil from said resistance component and said reactance component.
 26. An apparatus according to claim 22, wherein said film is a conductive film. 